The vapor-phase oxidizing dehydrogenation of methanol to formaldehyde, of isopropanol to acetone and that of sec.butane to methylethylketone are among the most widespread industrial processes.
Known in the art are two basic industrial processes for the production of formaldehyde, namely:
1. Vapor-phase oxidizing dehydration of metahnol in a mixture with air oxygen and steam on a catalyst comprising pure silver or silver deposited onto a carrier.
The process is conducted at a temperature within the range of from 500.degree. to 750.degree. C. in an excess of methanol relative to its stoichiometric amount under partial oxidation conditions with the use of alcohol-air mixtures, wherein the concentration of methanol is above the upper limit of explosiveness (cf. J. F. Walker "Formaldehyde" Goskhimizdat Publishing House, 1957; British Pat. No. 1,131,380 published 1967).
2. Vapor-phase oxidation of methanol in a mixture with air oxygen on an oxidized iron-molybdenum catalyst. In certain cases the catalyst contains, as an additive, oxides of other metals. The process is conducted at a temperature within the range of from 370.degree. to 430.degree. C. in an excess of air relative to its stoichiometric amount under conditions of incomplete oxidation of methanol using alcohol-air mixtures with a content of methanol below the lower limit of explosiveness (cf. J. F. Walker "Formaldehyde" Goskhimizdat Publishing House, Moscow, 1957; British Pat. No. 1,131,380 published 1967).
The process for the production of formaldehyde on a silver catalyst occurs at higher space velocities of methanol supply. The existing production plants feature high productivity, and are simple and compact in size. For this reason, construction of such plants necessitates small initial investment per unit product, and enables production of both concentrated methanol-water solutions of formaldehyde and aqueous solution of formaldehyde without no methanol. At substantially identical production costs of the resulting formalin this process features greater flexibility, ease of control and replacement of the catalyst as compared to the processes contemplating the use of an oxidized iron-molybdenum catalyst.
However, this process provides an insufficient yield of the desired product and a low degree of conversion of methanol mainly due to low activity and selectivity of the catalysts employed which have a short service life and are difficult to regenerate or are not regenerable. Formalin produced with the use of such silver catalysts frequently has an increased content of formic acid which impairs its quality, and the purification of formalin from formic acid on a special ion exchange unit substantially increases production costs.
In the production of formaldehyde by an incomplete oxidation of methanol on oxidized iron-molybdenum catalysts there is observed a higher yield of the desired product at a substantially total conversion of methanol. The catalyst employed in this process has a longer service life and a low cost, however, it cannot be regenerated. Due to the necessity of supplying large quantities of excess air and a relatively low space velocity of methanol, as well as the use of condensation equipment and a tubular reactor with a complicated system cooling of the contact gases, the specific capital investments are increased, thereby increasing the production costs. For this reason, the method for preparing formaldehyde on silver catalysts is the most widespread in the industry because it is profitable on high-capacity units due to lower unit capital investments.
The principal process flow-sheet for the production of formalin by oxidizing dehydrogenation of methanol in a mixture with air oxygen and steam in the presence of silver catalysts is substantially the same in the majority of chemical plants. The units for the production of methanolic formalin (with a content of formaldehyde of from 37 to 44% by mass, methanol--5 to 12% by mass and formic acid--up to 0.04% by mass) incorporate, as a rule, apparatus for contact oxidizing dehydrogenation of methanol and absorption of formaldehyde and the unreacted methanol from the contact gases by means of water. To obtain methanolic formalin with an increased concentration of formaldehyde (up to 50-55% by mass) and a relatively low content of formic acid (below 0.04% by mass), the units can incorporate apparatus for concentrating of formalin and ion-exchange purifiers. In the production of methanol-free formalin (with a content of formaldehyde of up to 50-55%, methanol--up to 1.0% and formic acid up to 0.04% by mass) the process units, as a rule, incorporate all the above-mentioned apparatus. The recovered unreacted methanol from the crude formalin in a rectification column is recycled back to the process after purification.
The process of contact oxidizing dehydrogenation of methanol to formaldehyde on silver catalysts under commercial production conditions is generally conducted in the following manner.
Methanol is fed into a mixer, wherein it is diluted with steam condensate or water to a concentration of from 65 to 90% by mass. After the mixer the resulting aqueous-alcoholic mixture is delivered into an alcohol-evaporator, whereinto air is fed which is preliminarily cleaned to remove foreign matter, and compounds of sulphur and ammonia. In the alcohol evaporator vapors of methanol and water are formed at a temperature within the range of from 50.degree. to 80.degree. C. which are mixed with water. After the alcohol evaporator the alcohol-air mixture has the following approximate composition: methanol--36.2%, water--15.3%, oxygen--9.6% and nitrogen--38.9% is fed into an overheater. In the overheater the mixture is heated to a temperature of from 100.degree. to 120.degree. C. and the resulting alcohol-vapor-air mixture is fed into a contact apparatus having passed through a fire-retarder.
In the contact apparatus, on a silver catalyst and at a temperature of from 500.degree. to 750.degree. C., methanol is converted to formaldehyde which is in a mixture with the following by-products: CO.sub.2, CO, H.sub.2, CH.sub.4 and HCOOH. The contact gases after the zone of contact with the catalyst are passed into the lower section of the contact apparatus to a subcontact cooler, wherein they are cooled to a temperature of from 70.degree. to 150.degree. C. Then the contact gases, while cooling in a heat-exchanger to a temperature not exceeding 20.degree. C., are passed into an absorption column, wherein they are sprinkled with water. As a result of absorption of formaldehyde and the unreacted methanol with water there is obtained methanolic formalin in the column still with the required content of the main product. The absorption gases formed in the absorption column with a specific content of CO.sub.2, O.sub.2, CO, H.sub.2, CH.sub.4 and N.sub.2 comprising a mixture of water insoluble products of side reactions of the oxidizing dehydrogenation of methanol to formaldehyde. Nitrogen and the unreacted air oxygen are combusted in a flare or vented to the atmosphere.
In the production of methanolic formalin with a low content of formic acid, the crude formalin at the outlet of the absorption column is delivered into an ion-exchange purification column, while in the production of methanol-free formalin with an increased concentration of formaldehyde and a lowered content of formic acid, the crude formalin after the absorption column is delivered into a rectification column or an evaporator and further into an ion-exchange purificaion column. The unreacted methanol distilled-off from the crude formalin in the rectification column is recycled back to the process.
The principal process scheme of the units for oxidizing dehydrogenation of isopropanol to acetone and sec.butanol to methyl ethyl ketone, and ethanol to acetaldehyde on silver catalysts is identical with the above-described flow-sheet of the production of formalin, except for individual process parameters and particularities associated with the recovery of the desired product.
Known in the art are processes for the production of formaldehyde by vapor-phase oxidizing dehydrogenation of methanol on the following kinds of silver catalysts:
(1) On a silver catalyst with the use of methanol instead of water to remove the heat evolved in the reaction. The process is designed for the production of formaldehyde in a yield of 91-92% as calculated for the reacted methanol and is carried out at a temperature within the range of from 500.degree. to 550.degree. C. The service life of the catalyst is extended from 60-90 days (in similar processes) up to 150-240 days. The process features a high content of methanol in formalin due to a low degree of conversion of methanol per run which does not exceed 70% by mass. The latter, in turn, explains a low yield of formaldehyde as calculated for the passed methanol which does not exceed 65% by mass.
(2) On a granulated silver catalyst at a temperature of 500.degree. C. To increase the yield of formaldehyde and ensure a low content of by-products, conversion of methanol is maintained at the level of 60% by mass. The excess of methanol is distilled-off in a rectification column and recycled, after purification, back to the process. The desired product withdrawn from the column still comprises a 37-50% aqueous solution of formaldehyde containing no methanol.
The process features a low degree of conversion of methanol, a low specific output, as well as the necessity of using a rectification unit. The employed catalyst comprises pure silver, which is expensive and has a short service life not exceeding 90 days.
(3) On a catalyst containing silver on a supporting screen made of stainless steel or an alloy of copper with 0.1-0.6% by mass of arsenic. In the former case the yield of formaldehyde as calculated for the passed methanol is 73.5% by mass, conversion is as high as 83% by mass; the time of stable operation or, accordingly, the service life of the catalyst is about 21 days. In the former case the yield of formaldehyde is 76.5% by mass, the degree of conversion is 84% by mass; the time of stable operation is 98 days.
The process features low yields of the desired product, reduced service life of the catalyst and low conversion of the starting feedstock.
(4) On a catalyst comprising crystalline silver. There are developed two processes for the production of formaldehyde on this catalyst. The first process is conducted at a temperature of from 600.degree. to 650.degree. C., wherein the degree of conversion of methanol is 77 to 87% by mass. The unreacted methanol is recovered by distillation and recycled. The second process is conducted at a temperature of from 680.degree. to 720.degree. C. The degree of conversion of methanol is about 97-98% by mass. The yield of formaldehyde as calculated for the converted methanol (selectivity of the process) is 89-91% by mass and similar in both processes. However, in the production of formaldehyde on said catalyst by the first process higher capital investments and operation expenses are involved due to the necessity of using process units for the recovery of methanol and ion-exchange purification for the removal of formic acid.
Both processes feature high material expenses, in the use of pure silver as the catalyst, and selectivity of these processes is insufficient.
Also known are processes for the production of carbonyl compounds by vapor-phase oxidizing dehydrogenation of C.sub.1 -C.sub.4 alcohols, in particular oxidizing dehydrogenation of methanol to formaldehyde, isopropanol to acetone, sec. butanol to methyl ethyl ketone and ethanol to acetaldehyde at a temperature within the range of from 500.degree. to 750.degree. C. in the presence of such supported silver catalysts as:
(5) "Silver-on-pumice" catalyst. The process features a relatively low yield of the desired product and insufficient conversion of the starting feedstock. The yield of carbonyl compounds as calculated for the passed alcohol does not exceed 76% by mass, the yield as calculated for the converted alcohol is not more than 92% by mass; the degree of conversion of the alcohol is 83% by mass. Due to low mechanical strength and wear-resistance of pumice, in the course of the production of carbonyl compounds there are observed considerable irreversible losses of silver which affects the catalyst service life which does not exceed 180 days. In this case the catalyst cannot be regenerated and after its use it is delivered to refining units for the recovery of silver. In the production of formalin there are frequently obtained aqueous solutions of formaldehyde with an increased content of formic acid (above 0.02% by mass) which impairs the quality of the final product, whereas the use of ion-exchange purification unit increases to the material costs of production.
(6) "Silver-on-alumina" catalyst. The catalyst comprises silver deposited onto alumina. The carrier can also incorporate, as an additive, oxides of alkali metals and rare-earth metals such as sodium, titanium, magnesium, zirconium, selenium and the like in an amount ranging from 0.5 to 5% by mass. However, due to a low selectivity of the process and the formation of considerable amounts of formic acid in the production of formaldehyde, this process has not been commercially implemented in the production of carbonyl compounds (cf. USSR Inventor's Certificate No. 106533, Cl. B 60 p 1/44, 1956).
(7) "Silver-on-carborundum" catalyst. The process for the production of methyl ethyl ketone by oxidizing dehydrogenation of sec.butanol in the presence of this catalyst features a relatively high selectivity (up to 98.5% by mass). In the oxidizing dehydrogenation of methanol to formaldehyde the process selectivity is at about the same level as in the case of the "silver-on-pumice" catalyst and equal to not more than 92% by mass. However, due to low degrees of the feedstock conversion, the yield of the target carbonyl compounds as calculated for the passed alcohol likewise in the process of preparation of formaldehyde and that of methyl ethyl ketone on this catalyst does not exceed 72.5% by mass. For these reasons, this process has also not acquired commercial implementation in the production of carbonyl compounds (cf. S. M. Lakiza et al. "Regeneration of Spent Silver Catalysts", Journal "Neftepererabotka i neftekhimija" ("Petroleum Refining and Petrochemistry"), 1974, No. 3).
Therefore, the above-described processes for the production of carbonyl compounds by a vapor-phase oxidizing dehydrogenation of C.sub.1 -C.sub.4 alcohols in the presence of silver catalysts both of pure silver and supported by a carrier, feature a low yield of the desired product per pass, low selectivity of the process and a short service life of the catalysts employed, as well as insufficient productivity of the process and the necessity of using ion-exchange purification units which increases the production costs and complicates the process scheme.